1. Field of the Invention
The present invention relates to the manufacture of pourable pulverulent rubber-filled mixtures starting with rubber solutions.
2. Discussion of the Background
In contrast to the manufacturing of molded pieces from thermoplastic materials, the processing of raw rubber material to manufactured rubber articles requires a complex production system which is costly in terms of time, energy, and personnel.
The pulverization of the raw rubber which is supplied in the form of balls, and the mixing with fillers, mineral oil softening agents, and vulcanization accelerators is accomplished on rolls or in kneader-mixers. Because of the frictional heat which develops, the mixing process must be carried out discontinuously, in a plurality of stages, where special requirements apply to the intermediate storage of the mixtures. After the possibly necessary step of processing with calenders or extruders, the final stage is carried out, which is vulcanization in presses or autoclaves.
To sidestep this costly and complex rubber processing, it has been proposed to use pourable filler-containing rubber powders. Using such powders, one can readily produce mixtures of rubber with, e.g., thermoplastic plastic powders, in a rapid and quasi-process.
Methods of producing pourable pulverulent mixtures of rubber and filler have been known for a number of years, particularly methods of producing mixtures of rubber and carbon black based on all-purpose rubbers (German Pat. No. 2,135,266, German AS No. 22 14 121, German OSs Nos. 23 24 009, 23 25 550, and 23 32 796, and German Pat. Nos. 2,439,237, 2,654,358, 2,822,148).
According to the state of the art methods described in these publications, pulverulent, filler-containing rubbers (so-called powdered rubbers) can be produced from rubber emulsions and rubber solutions, with the use of emulsifiers (cationic, anionic, or nonionic surface-active substances) and various precipitation aids (acids, metal salts, protective colloids, etc.), in single-stage or multistage processes.
However, these techniques have certain drawbacks from an economic, ecological, and process engineering standpoint. These include:
1. Use of costly chemicals, e.g., quaternary ammonium salts or alkylamine alkoxylates; PA1 2. Environmental pollution via electrolyte-containing waste waters; PA1 3. Use of costly precipitation techniques, e.g., maintenance of pH within specified ranges; PA1 4. Corrosion problems when acid precipitation media are used, which may lead to costly production downtime.
Other drawbacks include the use of appreciable amounts of emulsifiers and precipitation aids which become mixed into the rubber, where they can have a detrimental effect on various product characteristics.
There have been past attempts to produce a base mixture from a polymer and carbon black (German OS Nos. 21 47 429) by mixing a slurry of carbon black in an aqueous or organic solvent with a solution of the polymer in an inert organic solvent, and bringing this mixture into intimate contact with a gas stream of a temperature and speed such that the mixture is sucked up and dried, and the resulting base mixture is separated out. According to another state of the art method (German OS No. 21 54 422), a part of the volatile materials contained in a liquid mixture of an elastomer is converted to vapor in a flash evaporation step. Both of these methods result in crumbly, non-pourable products.
Finally, in the method according to German OS No. 22 60 340, a rubber solution obtained by polymerizing butadiene or copolymerizing butadiene with styrene in the presence of lithium catalysts is mixed with a filler dispersion. This flowable mixture is preferably heated to 100.degree.-200.degree. C., and then subjected to a pressure drop from relatively high to relatively low pressure, causing the organic liquid to undergo flash evaporation and the rubber-filler mixture precipitates out as a pourable, non-caking powder. This process is particularly suitable for processing hot rubber solutions obtained in adiabatic polymerizations. Where, after the polymerization reaction, the filler dispersion and rubber solution must be suitably heated, this method is not advantageous, because of the energy costs incurred and because of the additional thermal insult to the rubber.
In view of the stated disadvantages in the state of the art methods, there is a clear need for a method of producing pourable pulverulent filler-containing rubber, which is simple and inexpensive and excludes (and does not introduce) undesirable foreign substances.